Shot peening is a process whereby the physical nature of an existing device surface may be modified. In shot peening, solid particulate is propelled at high velocity by means of a carrier fluid either wet or dry, typically water and air respectively, so as to impact the surface of a target substrate typically a metallic substrate.
Shot peening has long since been established as a method to induce desirable stress properties in the surfaces of metallic devices wherein the impinging particles act as peening hammers causing a local plastic deformation at the surface rendering it less prone to cracking and corrosion. In addition to the significant pressures, large amounts of thermal energy, instantaneous temperatures as high as 1000° C. have been reported, are also generated locally at the surface in the vicinity of the impact. The limitation of shot peening processes involving the use of fluid streams to propel the shot particles to surfaces is that while shot may be recovered, large amounts of shot are required in the process itself. This places practical cost limitations on the quality of shot that can be used and a proportion of the shot used is consequently of irregular shape. The action of such irregular shot particles on surfaces can be abrasive in nature resulting in a degraded surface finish.
In addition shot peening nozzles are generally configured orthogonal to the surface so that the shot strikes the surface at a constant angle of incidence of close to 90°. As a result the resulting shot peened substrate surfaces are typically dimpled and uneven in texture.
Ultrasonic shot peening (Cheppe and Duchazeaubeneix, U.S. Pat. No. 6,343,495) has been developed as a means to circumvent these limitations. In this process shot is projected to the surface by means of a sonotrode. A sonotrode is a device that oscillates at ultrasonic frequencies and which impels the shot to the substrate surface. The sonotrode can be located in the bottom of a chamber such as a metal bowl. The rim of the bowl may contact the surface to be peened or the surface may form a wall or walls of a chamber that encloses the piece to be shot peened. This confinement of the shot means that much less shot is required as individual shot can repeatedly peen the work piece given that shot will continually rebound from the work piece and chamber walls back to the sonotrode from which it is again impelled to the work piece surface. The requirement for lower quantities of shot means that shot of higher material and geometry quality may be selected and a superior surface finish obtained. Additionally the random direction from which the shot impacts a work piece results in a surface less dimpled and less uneven in nature. Ultrasonic shot peening has largely been used for specialist applications and in particular for the work hardening of materials in the aerospace sector including for example turbine blades.
Shot peening and abrasive processes have been used extensively in surface science as a means to clean and condition surfaces in preparation for further treatments. A shot peening process is known for the simultaneous cleaning and painting of substrates (Kik and Schuurink—U.S. Pat. No. 4,517,248). The advantage being that the delay between cleaning and painting is eliminated minimizing re-oxidation of the cleaned metal surface prior to application of the paint. Gruss and Shapiro (U.S. Pat. No. 4,634,603) describe a process for the coating of printed circuit boards in which shot peening is employed to clean and condition the surface in preparation for subsequent coatings. A number of techniques have been disclosed which use shot peening or abrasive processes as a means to modify the surface chemistry/composition of metallic and other substrates by embedding desired solid material in the surface and these techniques may be broken into three distinct methodologies.
In a first methodology, a single type of single-phase solid particulate is used as the peening or abrasive media. In this method the shattered pieces of the particulate become embedded in the surface of the metal on impact. Such processes are mostly used to embed ceramic materials into a surface as the particles must have sufficient hardness size and mass to abrade or peen the surface. Examples include silica, alumina or calcium phosphate ceramics among others as, for example, disclosed in Arola and McCain (U.S. Pat. No. 6,502,442) and that of Kuo (U.S. Pat. No. 5,441,763).
The second methodology also involves the use of a single type of solid particle as the peening or abrasive media but the particles themselves are comprised of multiple components usually a hard component that gives the particle mass and density and a softer component that is desired to embed in or attach to the surface on impact. Examples are to be found in (Muller and Berger—US2004/158330; Bru—Maginez et al. U.S. Pat. No. 6,431,958; Hisada and Kihira U.S. Pat. No. 6,726,953; Omori and Kieffer—U.S. Pat. No. 6,015,586) and in the Rocatek™ bonding system for dental implants provided by 3M ESPE.
The third methodology is to employ a mix of different types of solid particulate media, a primary abrasive or peening material and a secondary material desired to embed or augment the surface in the same fluid stream so that they impinge the surface simultaneously. Examples of this process may be found in (Babecki and Haehner—U.S. Pat. No. 3,754,976; Chu and Staugaitis U.S. Pat. No. 4,552,784; Spears—U.S. Pat. No. 4,753,094; Vose—US20060089270; O'Donoghue. et al.—WO2008033867, O'Donoghue et al—US WO2009112945, where such processes are claimed to modify the surface composition of a variety of substrates with a number of materials including plastics, ceramics and metals. WO/2008/033867 teaches the use of grit blasting for the impregnation of metal oxide layers with solid particles delivered to the surface during a standard grit blasting treatment, the disruption caused to the surface oxide by the abrasive action allowing the smaller/softer solid particulate to become entrained in the oxide as it reforms.
Recently a fourth methodology for augmenting the surface of substrates, as detailed by Haverty and Kennedy—WO/2009/050251 (, has been described. This method involves atomising a liquid based precursor coating composition to form an aerosol, which is directed to the surface of the substrate in conjunction with a stream of hard, solid particles carried to the surface in a gas that may be of irregular shape (grit) but are more preferably of regular spherical shape (shot). In this process the collision energy released by the impacting shot mediates the transformation of the precursor composition into a well-adhered coating. This process solves problems associated with the prior art because the species augmenting the surface chemistry is not restricted to solid materials alone, the flow of augmenting species to the surface can be controlled precisely and those materials such as polymers particles that are typically prepared as colloidal suspensions can be used in the process. Both of these technologies suffer from the same limitation of regular shot peening processes with respect to quantities of shot used and consequently there exists the need for an improved process.